Precoat filter elements are frequently utilized in the treatment of liquids which contain both suspended particulate and dissolved chemical and ionic contaminants where the treated effluent must be of a very high degree of purity and closely adhere to specified standards of deionization or chemical composition. Such applications of precoated filters are found in the chemical processing industry, the treatment of industrial waste water, and the treatment of boiler feedwater in nuclear steam generating systems. Industrial processes utilizing steam from steam generating systems may produce condensate contaminated by corrosion products, by the inleakage of cooling water, and by various substances used in the process. Contaminants in the condensate or boiler feedwater of a nuclear steam generating system may cause corrosion and scaling of heat transfer equipment in the system, damaging heat exchange surfaces and decreasing their heat transfer efficiency. This, in turn, may result in overheating of tubes, followed by tube failure, further equipment damage, and possibly radioactive pollution of the environment. To prevent corrosion in nuclear steam generating systems, water treatment must include conditioning of the raw water supply, condensate returns from process steam or turbines, and boiler water.
A particular example of water treatment requiring stringent quality control, and in which the use of precoated filters is common, is the condenser stage of nuclear power plants. Two principal types of nuclear power plants are boiling water reactors and pressurized water reactors. Though they use different processes for generating steam to drive turbines in producing electricity and require different water chemistry, both employ similar water purification systems commonly referred to as condensate polishers. In boiling water reactors, should particles pass through the feedwater system and into the reactor, they may cause degradation and become radioactive. Should radioactive particles be created, they pose a costly disposal problem and may well present a threat of exposing personnel to radioactive materials. The presence of particulate contaminants in pressurized water reactors can cause stress cracks in heat exchanger tubes.
A raw water supply may initially contain many different types of dissolved and suspended matter. Most commonly, these materials include silica, iron, copper, calcium, magnesium, and sodium compounds. The metallic constituents generally occur in combination with bicarbonate, carbonate, sulfate, and chloride radicals. Many of these materials are ionic in solution, which may be used to advantage in treating the water to achieve a high degree of purity in the effluent.
As the water may contain a wide variety of harmful contaminants, it is common to use more than one technique in the treatment of the water to remove them. Usually, the water is first filtered to mechanically remove the larger suspended particulate contaminants and then demineralized through an ion exchange. The demineralization process, generally known as condensate polishing, can produce water closely approaching theoretical maximum chemical purity from ionic contaminants. Condensate polishing involves a reversible exchange of ions between the liquid phase and a solid phase which occurs by virtue of the charges carried by the ions. The solid phase typically comprises an ion exchange resin saturated with ionic groups which substitute new ions for ions present in the liquid when the solid is brought in contact with the liquid. In the condensate polishing process, ion exchange may be used to treat the contaminated water in two ways; ionic contaminants in the water may be replaced by relatively harmless products of deionization and, ionic contaminants may be transformed into products which are harmless or less harmful by replacing key ions in their molecular structure with other ions which in turn react with yet other ions present in the liquid to form the relatively harmless products. In the second case, for example, the replacement ions may react with remaining ionic contaminants to form additional water.
In one process for demineralizing boiler feedwater or steam condensate, for example, it is common to provide one tank, containing a resin saturated with hydrogen ions to replace metallic cations in the water, in series with another tank, to replace anions in the water. Each resin continues ionic substitution of contaminants until its deionizing capacity is exhausted. Once exhausted, the resin is either replaced, in the case of cartridge-tubular type condensate polishing systems, or regenerated to restore its ion exchange capacity, in the case of deep bed type demineralizer systems.
In condensate polishing systems used in the condensate stage of a nuclear power plant, for example, it is common to utilize a filter unit with precoat filter elements to simultaneously perform the filtration and demineralization steps of the water conditioning process. A filter element of this type of unit comprises a porous support structure, termed a septum, which is coated with a medium, termed a precoat, which performs both the filtration and ion exchange steps. In condensate polishing systems of the cartridge-tubular type, where the precoat is disposable, when the precoat becomes clogged with particulate contaminants, as evidenced by increase in pressure drop, or its ion exchange capacity is depleted, as evidenced by effluent water chemistry, it is discarded in a backwash operation in which the precoat is stripped off the septum and flushed out of the system. A new resin precoat is then applied to the septum.
A typical demineralization filter unit 100 used in such an application is shown schematically in FIG. 1. The filter unit 100 has a housing comprising a pressure vessel 101 which has an inlet 102 and an outlet 103. A tube sheet 104, is secured at its periphery to the inside wall of the vessel 101, and divides the vessel into a lower low pressure chamber, or plenum 105, and an upper high pressure chamber 106. The outlet 103 communicates the plenum 105 with the exterior of the vessel. The upper chamber 106 communicates with the exterior of the vessel via an aperture in the tube sheet 104 and the inlet 102.
Multiple filter elements 109 are located within the upper chamber 106. Each of the filter elements 109 includes a stand-off tube 111 which passes through a hole in the tube sheet. Each filter element 109 is supported by the tube sheet 104 and a seat 110 at the base of its stand-off tube 111. Each hole is circumscribed by a gasket (not shown) to provide a seal between the tube sheet and the stand-off tube. Each filter element 109 comprises a hollow core (not shown) and a porous septum 112 which can support a resin precoat. When the pressure in the upper chamber 106 is greater than that in the plenum 105, the porous septum allows water to flow into the core. The stand-off tube -11, extending vertically downward from the core serves as a discharge passage for filtrate from the element 109 into plenum 105.
The filter unit 100 of FIG. 1 incorporates perforated baffle plates 113, typical of such precoat filter units, in the upper chamber 106 of the pressure vessel 101, above the tube sheet 104 and over the inlet aperture 107. Thus, as water enters the upper chamber 106 through the inlet 102, the flow encounters the baffle plates 113 and flows throughout the upper chamber 106. The baffle plates are intended to reduce turbulence in the lower central portion of the chamber near the inlet, in particular, and throughout the chamber, in general.
Three basic, commonly used precoat filter septa are the yarn wound perforated core, coarse mesh, and porous metal cartridge types. Generally, these septa are of cylindrical configuration, though some may be of other configurations. Yarn wound perforated core septa typically comprise a perforated cylindrical stainless steel hollow core wrapped with string or yarn windings. Polymeric yarns, such as nylon or polypropylene, are commonly used. These yarns typically have a diameter of 1/16 inch and are wrapped around the hollow core to provide a septum depth of approximately 1/2 inch. Once the contaminated water has flowed through the precoat, it is intended to pass through the microporous openings of the yarn filaments rather than through any spaces between adjacent strands in the yarn winding. Coarse mesh septa include septa made of polypropylene mesh and wire mesh. Porous metal cartridges generally comprise a filtration medium of fine metal particulates sintered or otherwise bonded together. Coarse mesh septa and porous metal cartridges may be wound with yarn.
Precoats for these filters typically comprise a slurry of ion exchange resin particles suspended in a deionized water base. The suspension is formulated with a predetermined ratio of cation and anion particles, depending on the intended application.
The operation of the filter unit 100 of FIG. 1 may be divided into two stages: (1) a precoat stage and (2) a filtration stage. Generally, each filter septum 112 is precoated by introducing a flow of precoat resin slurry through the inlet 102 past the baffle plates 113 and through the filter elements 109 to accumulate a precoat layer on the upstream surface of the septum 112. The resin particle size distribution, flow rate, and proportion of flocculants are optimized to achieve proper precoat. The precoat layer is then compressed on the surface of the septum 112 of each element 109 by continuously circulating deionized water through the coated septum briefly at the process flow rate. A good precoat should exhibit an even thickness along the entire length of each of the septa, experience no erosion of the cake during precoat or treatment cycle, have no radial or axial cracks, and be of uniform thickness on all septa in the filter unit housing.
During the water treatment stage, the contaminated water flows into the vessel 101, through the inlet 102, past the perforated baffle plates 113 and into the upper chamber 106. The water contacts the resin precoat on the surface of each septum 112 and flows radially inward, first through the precoat and then through the septum 112. As the water flows through the resin precoat, the specially formulated ion exchange resin particles remove or transform minerals and other ionic contaminants in the water by the processes described above. The ion exchange precoat also acts as the filtration medium and typically has a finer pore structure than that of the septum 112. Ideally, particulate contaminants are captured in the precoat and prevented from penetrating into the septum 112. The treated filtrate then passes through the septum 112 to the core of the filter element 109, flows through the stand-off tube 111, into the plenum 105, and exits the vessel 101 through the outlet 103.
As the precoat continues to capture particulate contaminants during the demineralizing operation, the pressure differential across the filter unit 100 required to maintain a given flow rate increases until it reaches a level at which the filtration operation becomes too inefficient, and, as ion exchange continues, the ion exchange capacity of the resin is depleted, as evidenced by the change in water effluent chemistry. At that time, the treatment stage is discontinued, and the filter unit 100 is subjected to a backwash operation in which water is flushed through the unit in the reverse direction. The exhausted precoat is stripped off each septum 112 and flushed out of the system by the reverse flow. A new cycle is then begun with a new precoating operation.
Known demineralization precoat filter systems of this type have several defects. Generally, in precoat filter units of the prior art, uniform precoat of the filter septa 112 is not achieved. Despite the baffle plates 113, flow in the bottom of the vessel 101 is generally turbulent. This turbulent flow may result in erosion of the precoat over the lower portion of the septa. At higher flow rates, turbulent flow can be a cause of irregularity of the precoat thickness over the entire length of the element 109. Flocculating agents are generally added to the precoat slurry to promote the build-up of a thick but loosely packed permeable layer of precoat over the septum. In bottom inlet type vessels, a high inflow velocity is used during the precoat stage in an effort to obtain a good precoat layer at the top of the filter septa. Flow velocity decreases from the bottom toward the top of the elements. At the bottom of the elements, where velocity is highest, floc size tends to be very small. Over the mid-section of the element, velocity is lower and floc size is optimal. At the top of the element, flocs are much larger because of low velocity and the large oversize flocs produce a non-uniform precoat.
Many prior art precoat filter units are inefficient because backwash frequency is dictated by occurrence of the maximum allowable pressure differential prior to exhaustion of the precoat. The amount of contaminant which can be held within the precoat while maintaining the high flow rate necessitated by the limited flow area of the cylindrical septa within the differential pressure limitation is relatively small. The resin precoat, though clogged with particles, is not exhausted of its demineralizing properties and the precoated filter is utilized to filter less liquid than it could demineralize. Thus, backwashing frequently is dictated by the pressure differential, useful precoat is wasted, and the amount of radioactive waste which must be disposed of is increased.